Control circuit



Feb. 4,1941.

E. w. KELLOGG CONTROL C IRCUIT l Z g i560/vase 1 y :inventor Mttern@ Petented Feb; '4, 1941 PATENT OFFICE v2,230,299 coN'raoL CIRCUIT Edward W. Kellogg, Moorestown, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 30, 1937', Serial No. 151,269

Claims.

The present invention relates to control circuits, and more particularly to the type of` control circuits used in sound recording, telephony, radio broadcasting and the like, wherein one 5 component of a sound wave or a derivative thereof controls the sound wave, or a carrier'wave, or some mechanism acting in coordination with the wave.

It has been proposed in the past to provide,

for example, a, current corresponding to the envelope of a sound wave or a. current corresponding to the variations in amplitude of a carrier wave, or to some other characteristic of a wave or signal, either modulated or unmodulated, for

controlling apparatus rthrough which the wave passes. In connection with such devices, it is sometimes necessary to provide a delay circuit for transmitting the wave so that the control circuit can have the apparatus through which the wave is to pass properly conditioned for the wave on its arrival at the said apparatus.

Thedelay circuits used or proposed have been of many kinds including electrical, mechanical and acoustical. The procuring of the necessary delay by recording the sound and subsequently reproducing it from the record has also been proposed. It is evidentl that the production of such delaymust be accomplished with little or no distortion, since the control is to be exercised 3o on the delayed waves and unless the over-all result is better than that which is secured when such delay is omitted the purpose of the operation is defeated. The objection to recording the sound and subsequently reproducing it consists 35 in the distortion which practically all devices for recording and reproducing sounds introduce.

'Ihe conversion of electrical waves into mechanical or acoustical waves, and the re-conversion into electrical waves also involves the use of 40 translating devices such as loudspeakers and microphones which are not entirely free from distortion. For these reasons it would be desirable to keep the waves in the electrical form and secure the desired delay by transmitting them 5 over an artificial electrical transmission line.

The dimculty in employing this principal consists in the inherently high velocity of all electrical wave propagation, especially over types of transmission lines which are uniform.V Thus any uni- 50 form line providing the required delay time become extremely bulky and expensive. If the electrical transmission line is constructed of series coils and shunt capacities, thus providing what is known as a lumped line, the line will act as a low-pass flltervand eliminate all components having frequencies higher than a certain value. It is necessary that there be approximately three or more coils and condensers per wave length at the highest frequency which must c be transmitted. If this highest frequency is (Cl. Nil-100.1).

' taken as 10,000 cycles per second and it is desired to provide a delay of .05 second, for example, the line must carry '.05 10,000 equals 500 such waves at one time and in order thatit may not cut off waves of this frequency, at least 1500 coils and 1500 condensers must be employed.

Since the waves must traverse this very large number of units, afvery slight loss in each unit produces very serious over-all loss. Thus, if the loss is only slightly higher at high frequencies than at low frequencies, the over-all attenuation may be so much greater that it cannot be satisfactorily compensated in an amplifier. l

A similar difficulty is encountered in efforts to transmit waves through mechanical media such as torsional or other waves in solid bars of various shapes. In general, such bars have rates of propagation which depend on the frequency, and the losses also vary with frequency making it very diicult to transmit the Waves through a length of medium suflicient to produce the desired delay without serious distortion.

The most satisfactory results have been obtained with acoustic waves, or propagation through air, but even with this type of propagation, variations in attenuation with frequency become a serious factor, particularly in View of the fact that the highest audio frequencies which must be transmitted are of the order of 200 times the lowest required frequencies, for example, 10,000 and 50 cycles, respectively.

In accordance with my invention, I reduce the distortion which results from variations in the propagation characteristics of the medium, by employing waves in which the longest and shortest diier by only a relatively small percentage. If audio lfrequencies ranging up to 10,000 cycles per second are caused to modulate a Wave of 100,000 cycles per second, the result is a band of frequencies ranging from 90,000 to 110,000 cycles; thus the shortest waves are only of about 20% higher frequency than the longest. Therefore, if the medium attenuates high frequency waves more than low frequency waves, total variation will not be so serious as would be the case if the shortest waves had a frequency 200 times that of the longest waves. modulated high frequency waves, the transmitting and receiving device can be made to cause comparatively slight distortion since they are not required to deliver a range of frequencies large compared with the average frequencies.

In the performance of my invention I provide an oscillator which generates electrical waves of constant frequency (for example, 100,000 cycles per second). The acoustical waves to be transmitted are caused to modulate the output of this oscillator. 'Ihe resulting modulated high frequency electrical currents are applied to a high frequency device, which may be broadly tuned,

Moreover, by employing which radiates substantially plain Waves of frequencies in the range of 90,000 to 110,000 cycles. At a suitable distance from the sound radiating device, an appropriately designed microphone is placed in which are generated voltages corresponding to the high frequency waves radiated, but delayed by the time required for the waves to travel from the radiating device to the microphone. The output of the microphone is amplified and demodulated and the result is a replica of the original audiol frequency waves but with the desired delay.

It has been found that high frequency waves in air are sometimes attenuated so rapidly that it would not be practical to provide anyconsiderable delay by this method. Small quantities of Water vapor and carbon dioxide in the air result in very high attenuation. Attenuation is lowest of all if the high frequency sound waves are propagated in a monatomic gas such as argon. There is further advantage in employing a heavy gas, since this results in slower propagation andthus permits reducing the distance between radiator and the microphone. 'I'he heavy gas is also more effective in absorbing energy from the radiating device and imparting it in turn to the microphone.A The advantage of better coupling between the radiating and receiving devices andV the gas canbe further improved by employing gas under pressure.

Since there are advantages in transmitting the waves through a medium other than air, I propose, in one form of my invention, to employ a closed tube between the radiator and the microphone, the tube being filled with the desired gas, which may be above atmospheric pressure. In order to avoid interference between the direct waves and those refiected from the walls of the tubes, I propose to line the tube with the abe sorbent material, the tube being made of sum'- ciently large diameter to accommodate this absorbent material in addition to providing a clear opening having a diameter of the order of ten Y or more wavelengths of the transmitted waves.

One object of my invention is to provide a novel sound recording apparatus.

Another object of my invention is to provide a novel type of delay circuit,

Another object of my invention is to provide a novel combination of a transmission line and a control circuit therefor.l

Another object of my invention is to provide a novel type of volume compression or expansion apparatus.

Another object of my invention is to provide a novel type of ground noise reduction device for use in sound recording.

Other and incidental objects of my invention will beV apparent to those skilled in the art from a reading of the following specication and an inspection of the accompanying drawing in which Figure 1 shows schematically a form of my invention for volume compression in sound recording employing a delay circuit including a supersonic path in air,

Figure 2 shows a form of my invention as applied to ground noise reduction in sound recording, likewise using a supersonic air Path,

Figure 3 shows a form of my invention applied to sound recording for the purpose of volume compression, and ground noise reduction, and using a supersonic wave path in a tube which maybe lled with an appropriate gas,

Figure 4 shows a modified form of my invention using an acoustic air column for delay between lthe sound source andth pick-up microphone and a separate microphone for actuating the control circuit, and

Figure 5 is a longitudinal section of the input end of an acoustic transmission line.

Referring first to the form ofthe invention shown in Fig. 1. Sound is applied to the apparatus through an appropriate translating device I which is here represented as a ribbon microphone, although any other appropriate device for translating impulses into electrical energy may be used. Such devices include, for example, phonograph pickups, photoelectric pickups, etc.

`The output from this device is passed from the amplifier 2 where it is brought to a sufficiently high value and 'is then passed to the modulator 3. Supersonic waves of appropriate frequency are fed to the modulator 3 from the oscillator 4 and are modulated at 3 by the output from the amplifier 2 thereby producing modulated electrical waves of supersonic frequency. These waves are fed to an appropriate translating device 5 which is preferably a reproducer of the crystal or magnetostrictive type, although any other appropriate reproducer, such as an electromagnetic speaker or ribbon type speaker, capable of responding at the desired frequency may be used. The reproducer or radiator 5 should be capable not only of responding to the carrier frequency used, but

should have a suiciently wide range of response to transmit the side bands produced by modulation of the carrier. The output from the reproducer 5 is'directed to the microphone 6 by any appropriate means. The air column between the elements 5 and 6 may be conned by a tube or other enclosure, or Ait may be defined by the beam from a reflector or acoustic lens placed adjacent the reproducer 5 and focused upon the microphone 6, or the radiator or reproducer 5 may be so constructed as to produce substantially plane waves, in which case other concentrating or directing means may be omitted. The microphone 6, like the reproducer 5, should be capable of responding to the carrier frequency and the entire range of'` sideband frequencies. 'The output from the microphone 6 is fed to the amplifier and detector 1, where it is amplified to an approprlate value and then rectified to produce audio frequency waves corresponding to those impressed upon the translating device If The outputfrom this detector 1 is fed to the compressor 8. f

A portion of the output of the amplifier 2 is fed to therectier 9, where this output is rectied and applied as the control voltage to the compressor 8. In addition to rectifying devices proper, such, for example, as thermionic valves, the rectifier 9 contains a low pass filter to eliminate from the rectied currents such audio frequency components as may be present, Thus, therectiiied and ltered output of element 9 will be a smoothly varying current, having a magnitude Q dependent on the amplitude sof the audio fre-f be controlled be likewise delayed by a corresponding time interval.

Since compressor circuits are well known in the art, it is not described in detail here, and the same applies to several other elementary circuits,

such as the amplifier, modulator, oscillator, and detector.

In the compressor, the output from the rectifler 9 adjusts the contnol grid voltage to decrease the amplification on signals of large amplitude and to increase the ampliiication on signals of low amplitude, thereby compressing the volume range of the signals transmitted. Due to the delay in the filter circuit of the rectifier and due to the necessity for having the compressor prepared for the sound wave `before the sound wave reaches it, it is necessary that the grid voltage on the compressor be altered before audio frequency impulses from the microphone I reach the compressor, and the delay in thesupersonic air path is so chosen as to accomplish this result. 'I'he supersonic vibrations are propagated at substantially the samevelocity as ordinary sound waves, and a comparatively short air path is therefore su-iilcient to delay the sound kwaves until the compressor circuit is properly. conditioned to receive them. The period required for this delay not less than the delay in the unit 9 which includes a rectier and filter for smoothing out the sound waves.

After leaving the compressor 8, sound passes through the amplifier I to the recorder II which may be any appropriate type of sound recorder such as a photophonographic recorder or la. wax recorder.

The arrangement shown in Fig 2 differs from that shown in Fig. 1 only in that the control current is passed through the ground noise reduction amplifier and rectifier .I2 and from there to the shutter I3, which serves to obscure the unused portion of a sound record in the recorder II, as shown and described in McDowell Patent No. 1,855,197. vThe sound waves to be recorded pass through the modulator 3, the delay circuit 5, 6, the. detector 1, and to the amplifier described above, as shown in Fig. l. y

In the form of the invention shown in Fig. 3, the features shown lin Figs. 1 and 2 are combined. A portion of the output from the amplifier 2 passes to the modulator 3, through the supersonic delay circuit 5, 6, and the detector 1, to the compressor 8. Another portion passes .through the automatic volume control rectifier 9 to the compressor 8, as described above in connection with Fig. 1. A third portion of the output from the amplifier 2 passes to the ground noise reduction amplier and rectifier I2 and thence to the shutter I3, as described above in connection with Fig. 2. It will be apparent that in this arrangement the single delay circuit 5, 6 causes sufficient delay' in the transmission of the sound waves to permit the' functioning of both the compressor 8 and the shutter |I3. In- Fig. 3, the supersonic waves between the reproducer and the microphone 6, instead of being in free air, are 'shown as confined within the tube I4 in order to increase the emciency of the apparatus. This tube may be of any appropriate material and size, according to the time delay required.

Although it is permissible to employ a. tube of sumciently small diameter to avoid interference between the waves as propagated along the axis and as reflected from the'walls, I prefer to use a large diameter tube such that waves are propagated substantially as if in free space, which means thatthe tube must have a diameter -of a number of wave lengths, preferably of the order of ten or more wave lengths. In this manner, I avoid the attenuation which results from wall friction, which attenuation varies with frequency, and is of sucient magnitude to caus difficulties.

In order to avoid reections from the walls of the tube, which are quite serious in producing interference effects at supersonic frequencies, I prefer to line the walls of the tube with a suilicient layer of felt or other suitable material to prevent reflections of the sound waves within the tube. Likewise, in order to reduce attenuation, I prefer to use in the tube (instead of air) a gas of low viscosity, high molecular weight, and uniform attenuation, and high density or high molecular weight is of advantage in decreasing the length of the tube necessary, and thereby. decreasing the absorption, since the velocity of sound propagation inl a gas is inversely proportional to the square root of its molecular weight. The heavy gas has a further advantage in that it will absorb more energy from the sound radiating device 5 of Figs. 1, 2 and 3, and will better impart vibration to the microphone 6. A similar advantage is se cured by using gas under pressure, and the increased pressure affords a further advantage in reducing attenuation, for the reason` that pressure increases the density without increasing the viscosity.

The monatomic gas argon, which is readily available in quantity and has a -density about 1.38 vtimes that of air, is an especially favorable medium for the propagation of the supersonic Waves. l

In order further to improve the coupling between the gaseous propagation medium and the radiating and receiving devices 5 'and 6, I rnayv space may be permitted between the diaphragm and the plate in which the expansion passages are bored. Fig. 5 shows such a construction. A diaphragm I1 is vibrated at supersonic frequency by means of a piezo-electric crystal, magnetostrictive element, or any other suitable known means. Close to the diaphragm I1 is a plate I8, in which a number of tapered holes I9 are bored, in honeycomb arrangement. 'The waves, after expansion in they horns or tapered holes I9, recombine into a single plane wave front which is propagated along the tube I4. A similar arrangement may be employed at the` microphone 5.

The importance of thus increasing the coupling between the vibrating element and the propagation medium is not solely for the sake of increasing energy transfer, but also because most electroacoustic devices for loperation at supersonic frequencies are mechanically tuned, and the improved acoustic loading obtained by the horn The essential difa heavy gas, tends to broaden the tuning, and

which are designed'to bring' their several wave fronts into phase where they combine into a single plane wave. f

In the form' of the inventio'rshown in Fig. 4,

the microphonel and .amplifier 2 areused only.

to provide input for the rectier 9 which controls the amplification in 'amplifier I0, in order Ato automatically regulate the volume.v It will be apparent, of course, that this unit 9 may be connected to a volume compressor, to an expander,

to a ground noise reduction device, or to any device for which it is desirable that the lcontrol be capablefof anticipating a. change in level of the isound'waves. In this form of the invention, in order to secure a time delay, I provide' a pipe I5 through which the sounds to be recorded are transmitted. In this case, supersonic waves are not employed. The microphone I4, which is here shown as a pressure microphone, is located at an appropriate place along the pipe and connected thereto so that-the waves will reach the microphone I4 after a predetermined time delay- 'I'he portion of the pipe I5 beyond-the microphone I4 is lled with loosely packed tufts of felt or equivalent material, to provide acoustic damping and prevent the reection of sound waves.

In the form of the invention shown in Fig. .4, in which. the delayis obtained byproviding an acoustic path for waves of audio frequency, rather thanof supersonic frequency, the same advantages are secured by employment of heavy gases, or of gases under pressure, -as are obtained in the transmission of waves of supersonic frequency.

The employment of a heavy gas reduces the necessary transmission distance, thereby reducing the size of the apparatus, and reducing the attenuation of the waves. In reducing the total attenuation, the difference in attenuation of short waves as compared with long waves is also reduced, and distortion is thereby reduced. A'ttenuation is" also reduced by employment of a gas under pressure, whether this attenuation is the result of losses Within the gas itself or of friction against the sides of the tube.

One of the reasons why it is diicult to c onstruct radiating devices and sound pick-up devices or microphones free from distortion is that the vibrating elements are necessarily heavy compared with air, and it is diiiicult to avoid reso-fI nances in the mechanical structures.v Increasedr coupling between the diaphragm and the propagation medium,- obtained by employment of a denser medium, tends to damp the `oscillations of the radiating and receiving devices, and make it possible to secure more uniform response and freedom from distortion.

In Fig. 4, a heavier gas might be employed within the pipe I5 by providing a very light dia.- phragm across the open end. In order to employ a propagation medium in the delaypipe at other than atmospheric pressure, it would be necessary One such device is a parabolic reflector. Another consists in a large number of passages, .the lengths or contours yof to close the end in such a manner asto resist the pressure dilference. The most satisfactory method of employing a gas at'high 'pressure is to provide'an electro-acoustic device for producing the sound lin the tube, and to extend the pressure chamber soas to completely enclose both the radiator and the microphone. The sound radiator is, in this arrangement, supplied with electrical power from a microphone and amplifier. The system then becomes substantially equivalent to Fig. 3, except for the employment of acoustic frequency'radiators and microphones rather than instruments designed for supersonic frequencies,.

and the omission of the oscillator 4, the modulator 3, and the detector 1.

Having no w described my invention, I claim: 1. Meansfor translating sound waves into electrical currents, meansfor rectifying Lsaid electricalA currents,vr a`v second .means for translating said sound WaVesinto electrical currents, the

' second translation .occurring subsequently to said 4,iirst translation, and'means for controlling said second. translated currents with said rectified currents. v 2. A sound recording system comprising means f or translating sound waves intoelectrical currents, additional means for translating said sound `waves into electrical currents, means for delaying the second translation of said sound waves before translation thereof, and means for combining said currents into a common circuit for recording a sound record thereof.

3. A sound recording system comprising means forr translatingsound waves to be recorded vinto.

y electrical currents, means for dividing said currentsv into three portions, means fformodulating a carrier frequency with one of said portions, means for individually rectifying each of said other portions,A a supersonic delay circuit for said modulated portion, a detector for said supersonic w'aves, means for combining said detected supersonic waves with one portionf said rectified currents, and means for subsequently combining said lastmentioned combination' currents with the other portion of said rectified currents.

4. A` sound recording system comprising a source of sound waves, means for translating said sound Waves into electrical currents, a second means for translating said sound waves into electrical currents, means interposed between said SOuIe 0f Sound WafVeS and Said SeOlld transf' lating means for ydelaying the second translation of said sound waves, andmeansfor combining the currents of bothy of said translation means into a common circuit.

currents.

EDWARD W. KELLOGG. 

